Please refer to document 1 (U.S. Pat. No. 6,538,535 B2) and document 2 (Marco Guglielmi, Pierre Jarry, Eric Kerherve, Oliver Roquebrun, and Dietmar Schmitt, “A new family of all-inductive dual-mode filters”, IEEE trans. On Microwave theory & Tech., vol. 10, October 2001, pp. 1764-1769), the prior art provides a dual-mode waveguide filter 100 as shown in FIG. 1.
The dual-mode waveguide filter 100 has two dual-mode cavities 110, 120 coupled to each other. The dual-mode cavities 110 has an opening 111 for coupling with an input waveguide (not shown), and the dual-mode cavities 120 has an opening 121 for coupling with an output waveguide (not shown).
Instead of using circular or elliptical waveguide which is difficult to manufacture, the dual-mode waveguide filter 100 is designed as a rectangular waveguide with inductive discontinuities. The dual-mode waveguide filter 100 is called the all-inductive dual-mode filter. In the design of the all-inductive dual-mode filter, resonant frequencies of modes and coupling strengths between modes are controlled by the size of cavities and irises between cavities and input/output waveguide. The all-inductive dual-mode filter presents the advantage of being simple to design, simulate, and manufacture.
In addition, the all-inductive dual-mode filter exhibits high frequency selectivity since finite frequency transmission zeros can be generated inherently. The disadvantage of the all-inductive filters in documents 1 and 2 is that lots of physical parameters need to be carefully designed and adjusted since coupling topologies of filters are really complex (“Rosenberg, U. Amari, S., “Novel design possibilities for dual-mode filters without intracavity couplings”, Microwave and Wireless Components Letters, August 2002, pp. 296-298”, hereinafter being simplified by “document 3”).